This invention relates to antennas and, more particularly, to slot antennas which are fed from a microstrip transmission line.
Due to the high speed of modern aircraft and missiles, it is important that the size of protuberances from the surfaces of the craft be kept small, or possibly eliminated. Such craft usually have electronic equipment which requires antennas. Considerable work has been done toward reduction of the size of such antennas and mounting such antennas flush with the surface of the aircraft. Antennas employing a slot radiator are particularly useful for flush mounting as the slot is located flush with the skin of the craft and is backed by a cavity within the craft.
These slot antennas comprise a slot from which electromagnetic energy is radiated, a cavity, and a stripline probe for applying energy to the antenna in the cavity. The slot opening itself is usually not an actual opening which will create drag but rather is a sheet of dielectric material flush with the hull. The dimensions of the radiating slot antennas are generally determined by bandwidth and operating frequency. In order to obtain good directivity and gain, it is often necessary to employ an array of these slot antennas. The use of an array of slot antennas also lends itself to beam steering by controlling the phasing of the energy applied to the probe behind each slot.
One typical slot antenna is shown in FIG. 1 of the drawings and is the subject of U.S. Pat. No. 4,197,545, assigned to the assignee of this application. This antenna is made up of three boards, 10, 12 and 14. The top board 10 is made up of a dielectric material 11 having a layer of copper 13 bonded on the upper surface thereof, and includes a rectangular area 15 where the copper is etched away. Board 10 also includes a plurality of holes 17 located about the rectangular area 15 to define the boundaries of a cavity. The middle board 12 is made up of a dielectric material having a copper "T" section 18 bonded thereon with two holes 19 and 21 at the ends of the arms of the "T". Lower board 14 is a dielectric 23 having a copper layer 25 on the bottom. Boards 12 and 14 also have holes 17 therein aligned with the holes 17 in board 10. Holes 19 and 21 in board 12 are also aligned with like holes in boards 10 and 14. When the antenna is assembled, the boards 10, 12 and 14 are sandwiched together and fastened by, for example, gluing, using rivets, or other connecting means. Connecting pins are also placed through all of the holes 17 (and holes 19 and 21) in each of the three boards to connect the three boards mechanically and electrically. These connecting pins may be rivets or other fastening means, or plated-through holes of the type commonly used in printed circuit technology.
In operation, radio frequency energy is applied to the antenna via a path 27 to the top arms of the tee probe 18, which arms are terminated via holes 19 and 21 through the connecting pins mentioned previously. In actual operation the ends of the tee probe arms at the holes 19,21 will be at ground potential due to the rivets or plated-through holes which connect the arms to surface 13 of board 10 and surface 25 of board 14. When a slotted plate, such as plate 10, is excited by the radio frequency energy applied to path 27 of board 12, the slot acts analogous to a magnetic dipole antenna and radiation will be emitted from the dielectric area 15 on board 10. Further information regarding this type of antenna may be had by reference to said U.S. Pat. No. 4,197,545, the entire disclosure of which is incorporated herein by reference.
In an alternate embodiment of a slot antenna similar to this antenna, only two layers of printed circuit boards are used, with the middle layer 12 eliminated and instead the tee probe 18 is applied to the top layer of board 14. Such an antenna is described in U.S. Pat. No. 4,562,416, the entire disclosure of which is incorporated herein by reference.
These antennas provide excellent performance, however, they are expensive to construct and have problems when, after construction, changes have to made thereto. This is particularly true when a number of such elements as described in FIG. 1 are used together in an array. One of the problems with a multiple board antenna of these types is that when the layers are fastened together it is essential that there be no air pockets between the dielectrics, otherwise the characteristics of the antenna will change in an unpredictable fashion.
Another problem is that, if the antenna is already assembled and there is a problem such as the desire to add tuning stubs, etc., it is very difficult to disassemble the antenna. For example, if one wanted to add a tuning stub 29 (shown in dashed lines) to the feed 27 shown in FIG. 1, one would have to disassemble the antenna, add the tuning stub, and would not be able to determine the effect of the tuning stub until the antenna is reassembled. If the tuning stub does not provide the desired results, then it would have to be disassembled and adjustments again made. This might have to be done a number of times until the desired results are obtained.
For large antenna groups a minor error in only a single element could be corrected only by disassembly of the entire array.
Antennas using multiple layers are also thicker and more expensive to manufacture.
Accordingly, it is an object of this invention to provide an improved antenna.
It is another object of this invention to provide an improved slot antenna made up of only a single board.
It is a further object of this invention to provide an improved slot antenna in which all the elements of the antenna are exposed so as to permit easy access to the components to make changes to them.